Process of making membrane for removal of low density lipoprotein-cholesterol from whole blood

ABSTRACT

The present invention relates to the efficient removal of low density lipoprotein cholesterol complex (LDL-C) from whole blood. More specifically, it relates to the use of an immobilized affinity agent on a microporous plasmapheresis membrane. The immobilized affinity agent is polyacrylic acid bound directly and/or through an interaction with silica and/or calcium chloride to a microporous hollow fiber membrane.

This is a continuation-in-part of copending U.S. Ser. No. 618,791, filed Nov. 27, 1990.

TECHNICAL FIELD

The present invention relates to the efficient removal of low density lipoprotein cholesterol complex (LDL-C) from whole blood. More specifically, it relates to the use of an immobilized affinity agent on a microporous plasmapheresis membrane. The immobilized affinity agent is polyacrylic acid bound directly and/or through an interaction with amorphous silica and/or calcium chloride to a microporous hollow fiber membrane.

BACKGROUND

Atherosclerosis is the thickening and loss of elasticity in the inner walls of arteries, accompanied by the formation of small fatty modules on the artery walls and degeneration of the affected area. Atherosclerosis presented in the form of coronary heart disease and cerebrovascular diseases are major causes of morbidity and mortality in many industrial countries. Elevated plasma levels of low density lipoprotein-cholesterol complex (LDL-C) correlate with an increased risk for the development of atherosclerosis.

Patients at high risk for atherosclerosis are encouraged to make dietary changes in an attempt to control LDL-C levels. However, patient compliance is not always high and there is a large patient population which cannot control LDL-C levels merely through dietary modifications.

Drug therapy is also commonly used to try to lower LDL-C levels. While drug therapy is effective for many patients, there are still a large number of patients who are resistant to drug therapy or who suffer too many side effects to warrant its use.

In addition to dietary changes and drug therapy, attempts have been made to remove LDL-C directly from the plasma of patients through extracorporeal methods. These methods include plasma exchange, filtration based on molecular size, immunoadsorption, heparin precipitation and dextran sulfate adsorption. While these methods effectively remove LDL-C from plasma, they also remove varying quantities of desirable plasma components. The plasma exchange method removes all plasma and replaces the volume with plasma or albumin replacement solutions. All valuable plasma components, such as high density lipoprotein (HDL), and proteins are removed in addition to the LDL-C. The other methods, while better than plasma exchange, have varying degrees of specificity for only LDL-C. With filtration based on molecular size, there is considerable loss of proteins with molecular weights greater than 250-400 kD. Immunoabsorption is specific for LDL-C only, but its efficiency for removal of LDL-C is not as great as other methods. Heparin precipitation and dextran sulfate adsorption remove LDL-C, but a loss of 20-40% of HDL is generally expected; also the adsorbing capacities are fairly low. Since HDL plays an important role in reducing a patient's risk for atherosclerosis, a method which eliminates or minimizes the loss of HDL is highly desirable.

Previous filtration methods have also utilized carriers, such as agarose beads, which lack mechanical strength, and as a result are difficult to handle and operate. When fluid is passed through these carriers, there is a high probability of blockage. Additionally, these carriers may be destroyed by sterilization techniques. These carriers might also leach materials into the patient fluid.

Polyacrylate has been tested as a sorbent for lipoproteins from human plasma (Thies et al., Artificial Organs (1988) 12(4):320-324). Negligible loss of HDL and plasma proteins was shown with this absorbent. Polyacrylate has been attached to cellulosic beads through amide linkages. While the preparation was useful, it was not optimal for the treatment of whole blood. As mentioned previously, cellulosic beads do not have good mechanical strength, block easily, and are not easily sterilized.

Kuroda et al. (EP 0143369) describe a porous adsorbent for absorbing low density lipoproteins having a silanol group and a synthetic polyanion linked with the surface. To prevent clogging, the porosity of the adsorbent must be distributed over a broad diameter range. By contrast, the microporous membrane of the present invention has uniform pore diameters. Murakami (Japanese P.A. 01-229878) describes porous polyester fibers coated with methacrylic acid which are useful to remove bilirubin or LDL from body fluids. Sterilization of polyester fibers can be problematic. Kuroda et al. (Japanese P.A. 63-232845) describe an absorbent material having on its surface a synthetic linear polymer which has both a carboxyl group and sulfate or sulfonate groups.

To date, the majority of extracorporeal methods for the removal of LDL-C have involved two separate steps. First, the blood must be separated into cellular components and plasma components. This is usually done through centrifugation or filtration. Second, the plasma is treated to remove LDL-C. Finally, the treated plasma and cellular components are returned to the patient. The procedures are both time consuming and require a great deal of handling of blood products, which leads to increased potential for infections. Methods involving a closed system which are relatively rapid, efficient and require limited handling of blood are highly desirable.

SUMMARY OF THE INVENTION

The present invention provides high efficiency removal of low density lipoprotein cholesterol complex (LDL-C) directly from whole blood using an immobilized affinity agent on a microporous plasmapheresis membrane. LDL-C removal is achieved during the plasmapheresis process. The immobilized affinity agent is polyacrylic acid bound directly and/or through an interaction with silica and/or calcium chloride to a microporous polysulfone hollow fiber membrane.

In one aspect, the invention relates to the effective and highly specific removal of LDL-C from the plasma portion of whole blood. The invention removes negligible amounts of HDL or other blood proteins.

In another aspect the invention is superior to prior extracorporeal methods in that whole blood passes through one device where it is simultaneously separated into plasma and cellular components, the LDL-C is removed from the plasma, and the treated plasma and cellular components are returned to the patient. The process is approximately twice as rapid as conventional treatments and requires a minimum amount of blood handling.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram indicating the action of the device of the invention.

FIG. 2 illustrates graphically the ability of the device of the invention to lower total cholesterol and LDL-C without effecting HDL and free hemoglobin levels.

DETAILED DESCRIPTION OF THE INVENTION

A membrane has been discovered which has properties that are advantageous for the removal of the complex of low density lipoprotein and cholesterol (LDL-C) from whole blood or plasma. The polysulfone hollow fiber membrane has polyacrylic acid immobilized on its surface. The membrane has desirable mechanical and specificity characteristics for its intended purpose of LDL-C removal. The membrane can also be sterilized by autoclaving techniques.

The Membrane

The membranes of this invention are polysulfone-based polymeric compositions. Polysulfones are a known class of polymers which have been used to form various types of membranes. Polysulfone membranes are of a substantially non-flexible physical form. "Polysulfone", "polyarylsulfone", "polyether sulfone", and "polyarylether sulfone" are each intended to define a polymeric material having a combination of sulfone groups, aryl groups, and ether groups in the polymer chain and which may also contain alkylene groups therein. Polysulfone (PS) polymers are available in a variety of grades with respect to molecular weight, additives, etc. High molecular weight polysulfones may be preferred for preparation of membranes with additional strength. UDEL™ P-1700, and UDEL™ 3500 polysulfone polymers (Amoco Performance Products Inc.) are suitable. Other suitable commercially available polysulfones are under the tradenames of ASTREL (3M), VICTREX (ICI), and RADEL (Amoco). Polysulfone is used as the primary polymeric component of the membrane because of such beneficial characteristics as thermal stability, resistance to acid, alkali and salt solutions, high mechanical strength, etc.

The polysulfones found useful as membrane components of the present invention are polyaryl ether sulfones. The polysulfone can be viewed as having recurring units which is shown below: ##STR1## the SO₂ group may be in the ortho, meta or para position on the ring and where R represents ##STR2## wherein n is an integer of 0 to 3 (preferably 0 or 1 ) and each R' independently is selected from hydrogen or a C₁ -C₃ alkyl, preferably methyl. The above polyarylether sulfones may be used as homopolymers or as copolymers of the polymeric groups described above where R is selected from more than one of the groups described hereinabove. Further, the above polyarylether sulfones may be formed into copolymers with polysulfone groups which are void of ether groups therein such as: ##STR3## and the like. The homopolymers and copolymers described above can be used as the sole polymeric component or mixtures or blends of the homopolymers and/or copolymers can be used as the membrane component. The formation of blends provides polymeric component which can have customized properties. For example, it is known that increase in ether oxygen and/or alkylene groups in the subject polymers provides decrease in the soften temperature of the polymeric component and, therefore, aids in providing a composition which can be processed at a designed temperature. The subject polysulfones can be prepared by known manners.

The polysulfones used herein should have a weight average molecular weight of from about 20,000 to about 200,000, preferably at least about 50,000 to about 150,000. The polymer Tg will be dependent upon the structure of the polymer as described above and can be determined by one skilled in the art by conventional analytical means.

The subject polysulfones have benzylic hydrogens which can be independently substituted by nondissociative groups, such as alkyl (preferably C₁ -C₃ alkyl) or halogen (preferably chlorine) or by a dissociative group, such as sulfonic or carboxylic acid group. Each of the aryl groups may be unsubstituted or substituted with one or more of particular groups described above or may be substituted by different groups on a single aryl group or each on different aryl groups.

Other polymers or prepolymers can be used in combination with the polysulfone polymer, if desired, to impart various characteristics to the membrane product. Polyethylene glycol (PEG), polyvinyl pyrrolidone (PVP) or any of a variety of polyurethane prepolymers may be used with the polysulfone to prepare these membranes. Polymers or prepolymers are added to the polysulfone polymer in order to modify the structure and surface characteristics of the polysulfone membrane. The additional polymer or prepolymer becomes an integral part of the membrane structure.

A. The Casting Solution

The casting solution is a multicomponent solution comprising polymeric and solvent components. The primary polymeric component will be the polysulfone polymer. The polymeric component would, of course, also comprise any other polymer or prepolymer which is used together with the PS polymer to form the membranes. Where reference is made to the polysulfone solution or casting solution, it is intended to include all polymeric components. That is, it will include the polysulfone polymer and, where appropriate, it also will include a selected additional polymer or prepolymer as described above.

The solvent component of the casting solution must be one in which polysulfone (as well as any other polymer or prepolymer used) is soluble. The polysulfone polymer is soluble in various solvents, such as 4-butyrolactone, N-methylpyrrolidone (N-MP), dimethylformamide (DMF), N,N-dimethylacetamide (DMA), cyclohexanone, and chloroform. 4-Butyrolactone is the preferred solvent.

At least about 8.0 wt. % and up to about 35.0 wt. % polysulfone in solvent should be used, preferably about 8.0 to about 22.0 wt. %. Above 35 wt. %, it will be difficult or impossible to dissolve the polysulfone in the solvent. Below about 8%, precipitation will be too slow for formation of hollow fibers, and the fibers are too fragile to handle practically. Up to about 20.0 wt. % of a second polymeric component, that is, one or more of the polymers or prepolymers described above, can be added to the PS solution.

The casting solution can also contain silica. Silica can be present in amounts of about 0.1 to about 10% wt/wt, preferably about 5%. The silica aids in the immobilization of polyacrylic acid to the membrane during the next step of processing. Silica acts as a pore former and viscosifier to achieve a microporous structure with a nominal pore size of about 0.4 micron to about 0.65 micron. The casting solution can also contain polyacrylic acid (PAA). PAA can be present in amounts of about 0.01 to about 2% wt/wt, preferably about 0.5-1%.

B. Precipitation Solution

The precipitation or coagulation mechanism of membrane formation is affected by the composition of the precipitation solution as well as that of the casting solution, and the composition of these two solutions are interdependent. In this disclosure, the terms "precipitation solution", "coagulation solution," "quench solution," and "quench bath" are used interchangeably to refer to the solution in which the membrane is formed. For formation of hollow fiber membranes, both an outer and a center precipitation or quench solution will be employed. The solvent content of the precipitation solution controls the rate at which the solvent comes out of the casting solution. In turn, this controls the rate of increase of the polymer concentration to the point at which the polymeric component precipitates out of the casting solution to form the membrane. The same solvent usually is used in the casting solution and the precipitation solution. 4-butyrolactone and blends of 4-butyrolactone and N-methylpyrrolidone are the preferred solvents. Other solvents are discussed above with regard to casting solutions.

A non-solvent is often used in the precipitation solution in order to precipitate the polymer from the casting solution, thus causing formation of the membrane. For practical and economical purposes, it is preferred to use water as the non-solvent component of the precipitation solution. However, other non-solvents such as methanol, ethanol, propanol, butanol, ethylene glycol, acetone, methyl ethyl ketone, or the like, can be used instead of water, particularly when the solvent is water-immiscible. Alternatively, water and one or more other non-solvents can be used together.

In utilizing the method of this invention to prepare hollow fiber membranes, the precipitation solution used for the outer quench bath may be different from that used for the center quench fluid. In the preferred embodiment of this invention, the outer precipitation solution is water, and the center precipitation solution is 4-butyrolactone. Other solvents and non-solvents can be used as described above. In hollow fiber production, the center quench and outer quench are different phenomena. At center quench, a small volume of solution is used, which is almost in a static mode as compared with the casting solution. Conversely, the outer quench bath is present in large volumes and in a dynamic mode.

C. The Hollow Fiber Spinning Conditions

In preparing the hollow fiber membranes of this invention, a liquid-liquid or wet spinning process is used similar to that described in U.S. Pat. No. 4,970,034. That is, the casting solution is fed through an extrusion die (spinnerette) directly into a precipitation bath, while simultaneously introducing the center quench fluid through the central aperture of the spinnerette to mechanically maintain the hollow center hole of the fiber. The fiber is fabricated and simultaneously quenched as it is drawn through the precipitation bath. By using this wet-spinning process, fibers with homogeneous pore structure and membrane morphology are produced.

One of the key factors in preparation of the hollow fiber membranes of this invention is use of the wet spinning process; that is, spinning the casting solution under water. In addition, selection of appropriate solutions for the inner and outer precipitation baths is important, as is the appropriate drawing or spinning rate of the fiber as it is formed. The presence of the center quench fluid also allows for simultaneous polymer precipitation from both the inner and outer surfaces of the fiber. The spinning rate is adjusted to allow for exchange of components between the casting and precipitation solutions. The solvent is leached out of the casting solution and is replaced by the non-solvent from the precipitation solution. As a consequence, polymer precipitation occurs, leading to formation of the membrane.

Too rapid a drawing rate will cause breakage due to insufficient membrane formation to maintain membrane integrity or will cause elongation or deformation of the pores. Conversely, too slow a drawing rate will cause defects resulting from excessive pressure by the center quench solution, which may cause blow-outs in the fiber structure; also, non-circular fibers are produced. The preferred drawing rate will depend in part on the casting solution viscosity and temperature and in part on the factors described below. However, the drawing rate typically will be in the range of about 3.0 to about 30.0 feet per minute, preferably about 7.0 to about 15.0 feet per minute, and will produce round fibers.

The precise spinning conditions are adjusted in order to yield hollow fibers meeting the desired physical requirements of inner diameter and wall thickness. Centering of the central aperture of the spinnette is required in order to achieve a fiber having a uniform wall thickness. Any spinnerette suitable for the preparation of hollow fiber membranes may be used to prepare the membranes of this invention, however, quartz or glass spinnerettes are preferred in order to achieve the small inside diameters required of the hollow fibers of the invention. The spinning conditions left to be adjusted are the flow rate and pressure of the casting solution and the flow rate and pressure of the center quench fluid. These adjustments are well within the knowledge and ability of one of ordinary skill in this art. The preferred temperature for the casting solution will be in the range of ambient temperatures, although higher temperatures, e.g., up to about 70° C. may be employed to reduce the viscosity of the casting solution.

The dimensional and porosity characteristics of the membranes of this invention are such that LDL-C can pass through the fiber wall but most blood cells do not. Hemolysis occurs if numerous blood cells pass through the fibers, which is highly undesirable. However, passage of a small number of red blood cells through the fiber is acceptable. Generally speaking, membranes can be prepared which possess a pore diameter of between about 0.1 microns to about 0.7 microns, preferably between 0.4 and 0.65 microns. The inner diameter of the hollow fibers can range from about 150 to about 400 microns, preferably about 325 microns. The wall thickness can range from about ten to several hundred microns, preferably about 75 to about 100 microns.

D. Polyacrylic Acid Immobilization

Polyacrylic acid (PAA) is a semi-selective affinity agent for LDL-C. The presence of PAA on the surface of the PS hollow fiber membrane enables the effective removal of LDL-C from the plasma components of whole blood. Polyacrylic acid is immobilized on the surface of the fiber walls when the fibers are heated under pressure, preferably by autoclaving, for about 20 to about 40 minutes at about 120° to about 130° C. in a solution containing a caustic and PAA. In a preferred embodiment, the fibers are bathed in a PAA-containing solution and degassed under vacuum prior to the heat containing solution in amounts of about 0.01 to about 3.0% wt/wt, preferably about 0.5-2.0%. The caustic can be 0.3N to 2.5N sodium hydroxide, preferably about 1.0N to about 2.0N sodium hydroxide. This is a very simple and inexpensive means for anchoring PAA onto the surface of porous membranes for use as an affinity agent to effectively bind LDL-C. Without wishing to be bound by any theory, it is believed that the vacuum degassing step allows the PAA to be immobilized on both the outer and inner surface of the PS hollow fiber membrane. The membrane is more effective at removing LDL-C when the vacuum degassing step is performed.

PAA can be immobilized during the autoclaving step directly to the PS hollow fiber membrane or it can be immobilized indirectly through interactions with silica which can be added to the casting solution and embedded in the PS hollow fiber membrane. Greater amounts of PAA are immobilized to the membrane when silica is incorporated than without. While the actual nature of the interaction between PAA and silica is unknown, it is clear that silica enhances the quantity of PAA bound to the membrane. Prior to this invention, this enhancement was unknown. During this PAA immobilization step, the bulk of the silica is removed. This step also causes the fibers to be annealed and remain unaffected by subsequent autoclave steps. Fibers with silica are not microporous until the fibers are autoclaved in the base to remove the bulk of the silica.

Calcium chloride can also be added in or prior to this first autoclaving step to increase again the amount of PAA immobilized to the membrane, presumably by increasing the number of binding sites. While the actual nature of the interaction between PAA and calcium chloride is unknown, it is clear that calcium chloride enhances the quantity of PAA bound to the membrane. Calcium chloride is added to the first autoclave solution in an amount of 0.01 to 3% wt/wt, preferably about 1%.

E. Sterilization/Cleaning

The membrane of the invention is treated in a manner to ensure that it is sterile, the fibers are annealed, and also that no trace of residual solvent is present in the final membrane to reduce any chance of solvent or unsterile products leaching into the patient. For sterilization/cleaning the membrane is autoclaved a second time for about 20 to about 40 minutes at 120° to 130° C. in deionized water. The membrane can be optionally vacuum degassed prior to this autoclave step also. The membrane is washed again in water and soaked overnight in a water bath at ambient temperature containing about 5 to about 20% glycerine. This sterilization/cleaning process removes residual amounts of solvent and non-immobilized PAA. Unbound calcium chloride is removed by chelation. It is important that all calcium chloride is bound or removed by chelation to ensure that the membrane is not hemolytic and does not cause complement activation.

It is important to note that, if the fibers are autoclaved first in water, then in PAA, calcium chloride, and base, no PAA is incorporated in the membrane.

The Device

The membranes are dried, preferably at room temperature in air containing less than 50% relative humidity to remove excess water. The fibers are then placed in a housing, and both ends of the fiber are potted in place in the housing. The preferred housing is a FOCUS™70 housing (National Medical Care, a division of W. R. Grace & Co.-Conn.) which is packed to about 42% packing density with about 1,200 fibers per housing. Other convenient hollow fiber housings may be used.

Use

The membranes and the device of this invention are excellently suited for removal of LDL-C from whole blood or plasma. FIG. 1 is a schematic representation of the mechanics involved in using the LDL-C removal device of the invention. Whole blood is removed from the patient, typically from a vascular access point in arm 10 using suitable blood removal apparatus 14. Some suitable apparati for blood removal include hypodermic needles, fistulas, subclavian catheters or other in-dwelling catheters. The blood passes from blood removal apparatus 14 into whole blood tubing 16 and is pumped via optional blood pump 18 into LDL-C removal device 28. As whole blood is pumped through the lumen of the hollow fiber membrane of LDL-C removal device 28, plasma is forced through the channels of the microporous fibers and separated from the cellular components of the blood. The plasma is treated in LDL-C removal device 28 exiting via plasma exit port 30. The remaining blood components (high hematocrit blood) passes down through the lumen of the membrane(s) and out exit port 34. The treated plasma is pumped via optional plasma pump 32 through plasma tubing 36 and is reunited with the high hemocrit blood at junction 44. The whole blood is then returned to the patient along with additional saline 38 added through saline tubing 40 at junction 46 as necessary via return tubing 42 to suitable blood return apparatus 12. The pressure is monitored by monitor 20 before blood enters LDL-C removal device 28, while blood is in LDL-C removal device 28 by monitor 24, and as blood exits LDL-C removal device 28 by monitor 22. Pressure can be adjusted as necessary using blood pump 18.

Within the LDL-C removal device the action is as follows. The nominal pore size of the hollow fiber is such that it will reject or prevent the passage of blood cells through the membrane, yet permits the free passage of plasma and specifically the high molecular weight components such as LDL-C (2-6 million Daltons) through the membrane wall structure. As the plasma passes through the wall of the membrane, it comes into direct contact with the affinity agent PAA, and LDL-C is bound to the wall surface. The plasma which exits through the outer surface of the membrane contains less LDL-C. In a single step, the hollow fiber cartridge separates the plasma from the blood, removes the LDL-C from the plasma, and returns both plasma and blood components to the patients. Under normal operating condition (flow rate (Q)_(Plasma) ≦.2Q_(inlet) and transmembrane pressure (TMP)<40 mm Hg), the cartridge is saturated with LDL-C in about 30-40 minutes. The cartridge can be substantially regenerated with a 1.0M salt wash. This substantial regeneration represents about 85-95% of the original binding capacity restored.

In many of the devices of the prior art, an arterial/venous fistula must be implanted in the patient prior to treatment in order to achieve blood access to support the required higher flow rates for the devices. The access is often in the form of a subclavian catheter and the implant procedure is very invasive. The implant procedure carries certain risks with it as well, such as increased chance of blood clots. The device of the present invention does not require such high flow rates, and therefore conventional direct intravenous therapy type vascular access is possible. This procedure is much less invasive and has fewer risks associated with it. The flow rates of the device of this invention are optimal when plasma outlet flow is maintained at equal to or less than 20% of the blood inlet flow rate and when the pressure difference between the blood inlet and plasma outlet (TMP) is maintained at less or equal to 40 mm Hg. Back pressure is maintained on the plasma outlet flow to prevent hemolysis in accordance with standard procedures for plasmapheresis membranes.

The membranes and device of this invention dramatically reduce the amount of LDL-C from whole blood or plasma. A significant quick reduction in LDL-C levels is advantageous for some patients and cannot be obtained using drug or dietary regimens. The present device also drops LDL-C levels very selectively and effectively which is not necessarily the case for prior art devices. The invention further can facilitate plaque regression of atherosclerotic lesions insofar as reduction of circulating LDL-C levels permits.

This device is useful for reducing LDL-C in any number of increased cholesterol disorders. The primary candidates for use of the device of the invention include young individuals homozygous for familial hypercholesterolemia who have a family history of heart disease, patients with severe coronary artery disease that are non-operable, and all potential bypass candidates. The most significant and acute cholesterol disorder is hypercholesterolemia and treatment of this disorder is certainly applicable to the device of the invention.

EXAMPLES

The following examples are intended to illustrate but not to limit the invention. The following abbreviations have been used throughout in describing the invention.

    ______________________________________                                         dl        deciliter(s)                                                         °C.                                                                               degrees centrigrade                                                  Q         flow rate                                                            g         gram(s)                                                              HDL       high density lipoprotein                                             hr        hour(s)                                                              kD        kilodalton(s)                                                        l         liter(s)                                                             LDL-C     low density lipoprotein cholesterol complex                          m         meter(s)                                                             ml        milliliter(s)                                                        min       minute(s)                                                            M         molar                                                                %         percent                                                              PAA       polyacrylic acid                                                     PS        polysulfone                                                          psi       pounds per square inch                                               rpm       rotations per minute                                                 T.C.      total cholesterol                                                    TMP       transmembrane pressure                                               ______________________________________                                    

EXAMPLE 1

A particular membrane of the invention having polyacrylic acid and silica bound to the polysulfone hollow fiber membrane is prepared as follows. Polysulfone, 210 g (UDEL 1700, CAS #25135-51-7), was added to 1690 g of 4- butyrolactone (Kodak, CAS #96-48-0), in a glass jar with a sealable top containing a TEFLON, polytetrafluoroethylene (or other inert) liner. The mixture was rolled continuously on a roller mill for 48-72 hours at room temperature until the polymer was dissolved. To this solution of polysulfone in 4-butyrolactone was added 100 g of silica (SYLOX™ 2, Davison Division of W. R. Grace & Co.-Conn.). The jar was resealed and rolled continuously on the roller mill for at least 16 hours at room temperature to disperse the silica particles. This gave a casting solution that was 10.5 wt% in Polysulfone, 5 wt % in SYLOX-2 and 84.5 wt % in 4-butyrolactone.

The casting solution was then centrifuged at 2,000 rpm for 10 minutes to settle any poorly suspended silica particles. Next, the casting solution was pumped through a 40 micron stainless steel screen at 60 psi of pressure with dry nitrogen gas as the source of the driving pressure. After filtration the casting solution was de-gassed under mechanical vacuum at less than 10 mm Hg for at least 15 minutes and put in a stainless steel kettle that could be pressurized for delivery of casting suspensions to nozzle. No substantial solvent was lost during this degassing procedure due to the low volatility of the solvent. Under 60 psi of dry nitrogen gas, the casting solution was extruded through a glass nozzle within an orifice under the surface of a bath of deionized water. The core liquid of the spinnerette was 4-butyrolactone, driven by 40 psi dry nitrogen gas. The hollow fiber fabricated from the process during the under water spinning process was collected on a revolving wheel partially submerged under water. When the appropriate number of fibers were collected (800-1,200 revolutions), the fiber bundle was removed from the wheel, cut to chosen lengths, and soaked 16 hours at room temperature in deionized water.

Polyacrylic acid (CAS #9003-01-4) was immobilized on the fibers in the following process. Three (3) bundles of fibers of 8-inch length, each containing 800 fibers, were placed in 400 ml of a solution of 1.0N sodium hydroxide solution containing 1% polyacrylic acid (Aldrich, M.W.=250,000) in a stainless steel tray. The fibers in this solution were placed in an autoclave and heated under pressure at 130° C. at 30 psi for at least 30 minutes. The fibers were then removed from the NaOH/PAA solution and placed in 400 ml of deionized water and autoclaved again for 30 minutes at 130° C. at 30 psi of deionized water. The fibers were then soaked 16 hours at room temperature in a water bath containing 20% glycerine. After soaking in the glycerine bath the fibers were removed and allowed to air dry for 24 hours at room temperature. The dried fiber bundles were placed in the proper size device, and both ends were potted in place with a biomedical grade epoxy-resin system (Emerson & Cummings, Division of W. R. Grace & Co.-Conn., Cat #674A and 674B) as per instructions. The fiber device was now ready for testing after the excess fiber and potting compounds were trimmed from both ends. Once the device was tested to ensure the microporous membranes maintained pressure as expected, it was ready to be used for removal of LDL-C from plasma and/or whole blood.

EXAMPLE 2

Another membrane of the invention was prepared as described in Example 1 with the following differences. The casting solution contained 10.25% Polysulfone instead of 10.5%. The first autoclave step contained 0.5% polyacrylic acid and 0.8% calcium chloride.

EXAMPLE 3

A hollow fiber device containing 400 fibers as prepared in Example 1 with a surface area of 226.0 cm² and total wall volume of 3.0 ml. was perfused with plasma from a 100 ml reservoir of high cholesterol human plasma. The recirculation of high LDL-C plasma through the device was maintained at a flow rate of 38 ml/min giving a shear rate of 550 sec⁻¹ to achieve a steady plasma filtration rate through the walls of the fibers. Plasma samples were taken from the plasma exit port and filtrated at time 0, 30 minutes, and 60 minutes. The average transmembrane pressure (TMP) remained constant throughout the run at 90 mmHg, with the inlet pressure 95 mmHg and the outlet pressure 80 mmHg. Plasma filtrate flux values were 15.9 1/hr/m² at 30 minutes and 21.1 1/hr/m² at 60 minutes. Total cholesterol assays were performed on the plasma reservoirs using standard calorimetric assays (Sigma, No. 352-50) and nephelometry (Beckman Auto ICS Catalog Number 449310) to determine the level of the LDL-C associated protein apolipoprotein B.

The total cholesterol (T.C.) level was reduced from an initial value of 321 mg/dl to 241 mg/dl. The apolipoprotein B concentration was reduced from 152 mg/dl to 50 mg/dl. The albumin levels, also determined on the Beckman ICS nephelometer remained unchanged at 2,600 mg/dl. The difference in the pre- and post-total cholesterol values was used to determine the amount of T.C. removed from the plasma reservoir and a drop of 25.2% was observed. This corresponds to a binding of 27 mg total cholesterol per ml. of fiber wall volume.

EXAMPLE 4

A hollow fiber device containing 1,200 fibers described in Example 2 with a surface area of 2,215 cm² and a total wall volume of 20.9 ml was perfused with a 400 ml reservoir of human whole blood. The recirculation of the whole blood through the device was maintained at a flow rate of 69.0 ml/min giving a shear rate of 270 sec⁻¹. Whole blood samples from the reservoir and plasma samples from the filtrate were taken at time 0, 15 minutes, 30 minutes, and 60 minutes. The transmembrane pressure was maintained at 30 mmHg with the inlet filtrate flux values were 1.9 1/hr/m² at 60 minutes. Total cholesterol assays were performed on the samples using standard calorimetric assays (Sigma, No. 352-50) and nephelometry (Beckman Auto ICS, Catalog Number 449310) to determine the level of the LDL-C associated protein apolipo-protein B.

The total cholesterol (T.C.) level was reduced from an initial value of 450 mg/dl to 339 mg/dl. The apolipoprotein B concentration was reduced from 267 mg/dl to 146 mg/dl. The total protein level was reduced from 7.1 g/dl to 6.6 g/dl. After corrections for dilutional effects, non-specific absorption, and reservoir size, the total cholesterol level was lowered by 24.7% which corresponds to a binding of 9.4 mgs of total cholesterol per ml of fiber volume.

EXAMPLE 5

The procedure and device described in Example 4 was used with the following differences. The device was perfused with 500 ml of reconstituted human blood. The reconstituted human blood consisted of concentrated plasma containing high LDL-C levels mixed with fresh human blood cells. Whole blood samples from the reservoir and plasma samples from the filtrate were taken at time 0, 30 minutes, 60 minutes, and 90 minutes. Standard assay procedures were used to determine levels of total cholesterol, LDL-C, HDL, and free hemoglobin.

The results shown in FIG. 2 indicate that the device is capable of lowering cholesterol to acceptable ranges in a very short time without significantly effecting HDL levels. Free hemoglobin only increased 19 mg/dl over the course of treatment. This result indicates that the fibers are non-hemolytic since a value of less than 25 mg/dl is considered non-hemolytic.

EXAMPLE 6

Fibers were treated with and without vacuum degassing in order to determine its effect on the LDL-C removal ability. Membranes were prepared as described in Example 1 with the following differences.

For Fibers A, 4 bundles of 1200 fibers were placed in 1600 ml of a solution containing 0.5% PAA, 0.8% CaCl₂ in 1.0N NaOH in a stainless steel tray. The fibers were then directly autoclaved and treated as described in Example 1.

For Fibers B, the autoclave step which immobilizes the PAA to the fiber was preceded by a vacuum degassing step. Fibers B were treated exactly as Fibers A, except that the fibers were vacuum degassed as follows prior to autoclaving. Fibers B were placed in the PAA, CaCl₂, NaOH solution and degassed under pressure at 25° C. at about 30 inches Hg for 2 minutes. The vacuum was then released briefly and repeated for another 2 minutes at about 30 inches Hg. The fibers were then autoclaved as described in Example 1. Fibers B were then placed in 1600 ml deionized water, vacuum degassed as described above and autoclaved again for 30 minutes at 130° C. at 30 psi. The fibers were then assembled into devices as described in Example 1.

The hollow fiber devices containing Fibers A and Fibers B were tested by being perfused with 300 ml volumes of human high cholesterol plasma. Recirculation of plasma through the devices was maintained for 60 minutes at flowrates of 65 to 70 ml/min. Plasma samples for analysis were taken at 0, 30, and 60 minutes from both the plasma reservoir and the filtrate stream. Other operating parameters and assay methods used were identical to those described in Example 3.

The average total cholesterol concentration of the plasma pools used to test the undegassed devices (Fibers A) was reduced from 426 mg/dl to 335 mg/dl in 60 minutes, a 21.4% reduction Whereas the average total cholesterol concentration of the pools perfusing the degassed devices (Fibers B) was reduced from 436 mg/dl to 286 mg/dl in 60 minutes, a 34.4% reduction. Vacuum degassing, therefore, improved cholesterol binding of equivalent hollow fibers treated with an identical immobilization solution by roughly 35%.

The principles, preferred embodiments and modes of operation of the present invention have been described in the foregoing specification. the invention which is intended to be protected herein, however, is not to be construed as limited to the particular forms disclosed, since these are to be regarded as illustrative rather than restrictive. Variations and changes may be made by those skilled in the art without departing from the spirit of the invention. 

We claim:
 1. A process for preparing a membrane which binds low density lipoprotein cholesterol comprising(a) preparing, in a solvent for polysulfone, a casting solution comprising about 8 to about 22 wt. % of a polysulfone polymer, (b) preparing an outer precipitation solution comprising a non-solvent for polysulfone, (c) preparing a center precipitation solution comprising a solvent for polysulfone, (d) providing a precipitation bath containing said outer precipitation solution and having a hollow fiber-forming spinnerette partially immersed therein, (e) extruding said casting solution and said center precipitation solution through said spinnerette directly into said precipitation bath to form an extruded hollow fiber membrane, (f) drawing said extruded hollow fiber membrane through said precipitation bath, (g) submerging said hollow fiber membrane in a caustic solution comprising polyacrylic acid; (h) immobilizing polyacrylic acid to said hollow fiber membrane by heating under pressure the submerged fibers of step (g); and (i) annealing the hollow fiber membrane of step (h) by heating under pressure in water.
 2. The process of claim 1 wherein the submerged membrane is vacuum degassed between step (g) and step (h).
 3. The process of claim 2 wherein the membrane is vacuum degassed in water between step (h) and step (i).
 4. The process of claim 1 wherein silica is added to the casting solution during step (a).
 5. The process of claim 4 wherein about 0.1 to about 10% wt/wt silica is added.
 6. The process of claim 1 wherein calcium chloride is added during step (g).
 7. The process of claim 1 wherein the membrane has a nominal pore size of 0.4 to 0.65 microns.
 8. The process of claim 1 wherein the inside diameter of the hollow fiber membrane is between about 150 and about 400 microns.
 9. The process of claim 1 wherein the solvent for polysulfone is 4-butyrolactone.
 10. The process of claim 1 wherein the non-solvent for polysulfone is water.
 11. The process of claim 1 wherein the caustic solution is about 0.3N to about 2.0N sodium hydroxide.
 12. The process of claim 1 wherein said caustic solution comprises about 0.1 to about 2.0% wt/wt polyacrylic acid.
 13. The process of claim 1 wherein the conditions of step (h) are 130° C. at 30 pounds per square inch.
 14. The process of claim 1 wherein the water annealing of step (i) cleans and enlarges pore size.
 15. The process of claim 1 wherein a second polymer or prepolymer is added to the casting solution of step (a). 